The present invention generally relates to chamfering or edge rounding machines and more particularly, to an edge rounding machine for rounding edges of a floppy disk head or the like.
As shown in FIG. 1, a floppy disk head (workpiece) A is made of fragile material such as ceramics, etc. and is substantially formed into a rectangular parallelopiped shape. The floppy disk head A has a machining face 1 confronting a floppy disk (not shown). In order to prevent the floppy disk from being damaged by the machining face 1, an outer edge portion 2 of the machining face 1 is subjected, throughout its periphery, to curved chamfering or edge rounding. Thus, the machining face 1 has a flat portion 3, a curved chamfered portion or an edge-rounded portion 2a and a boundary 2b between the flat portion 3 and the edge-rounded portion 2a. The boundary 2b is required to be machined smoothly with high precision.
A prior art edge rounding machine for performing such edge rounding with high precision is shown in FIGS. 2 and 3a. The known edge rounding machine includes a grinding device 6. The grinding device 6 is provided with a rotary disk 14, an elastic sheet 4 and a grinding sheet 5 secured, through the elastic sheet 4, to an upper face of the rotary disk 14. Meanwhile, the workpiece A is rotated, pivoted and, at the same time, urged towards a grinding face 7 of the grinding device 6 along a rotary axis P' of the workpiece A so as to be subjected, in pressing contact with the grinding face 7 of the grinding sheet 5, to edge rounding. The known edge rounding machine further includes three holders 8 each for holding the workpiece A and a motor 9 for rotating the holders 8. The holders 8 are driven by the motor 9 so as to be each rotated about the rotary axis P' such that the workpieces A are each rotated about the rotary axis P'. The holders 8 are mounted on an end portion 10 of an L-shaped pivotal arm 12 so as to be each pivoted through a predetermined angle .alpha.' in response to pivotal movements of the pivotal arm 12. As a result, the workpieces A are pivoted about a pivotal centerline Q'. Reference numeral 13 represents a crank mechanism for pivoting the pivotal arm 12.
Furthermore, the holders 8 are axially movably supported by the end portion 10 of the pivotal arm 12 and are each urged downwardly by a spring 11. As a result, the workpieces A are each urged towards the grinding face 7 along the rotary axis P' so as to be brought into pressing contact with the grinding face 7. Since the known edge rounding machine is provided with three holders 8 mounted on the end portion 10 of the pivotal arm 12 as described above, three workpieces A can be machined simultaneously by the known edge rounding machine.
The prior art edge rounding machine of the above described arrangement has the following drawbacks (1) to (4).
(1) In the known edge rounding machine, when the workpiece A is disposed at a pivotal position most adjacent to a vertical line V' as shown by imaginary lines in FIG. 3a, the outer edge portion 2 of the machining face 1 to be subjected to edge rounding is machined into the edge-rounded portion 2a having an oblique curved surface. At this time, since the elastic sheet 4 disposed between the rotary disk 14 and the grinding sheet 5 is properly deformed elastically, an angle .theta. (FIG. 1) of inclination of the oblique curved surface of the edge-rounded portion 2a can be minimized, so that the edge-rounded portion 2a can be smoothly connected, at the boundary 2b, with the flat portion 3. Accordingly, when proper elastic deformation of the elastic sheet 4 is prevented due to its initial nonuniformity in coefficient of elasticity, surface hardness, etc. or change of its physical properties with time, it is impossible to smoothly connect, at the boundary 2b, the edge-rounded portion 2a and the flat portion 3. Furthermore, a problem arises in that since the angle .theta. of inclination varies due to change of coefficient of elasticity of the elastic sheet 4, quality of the finished product of the workpiece A becomes unstable.
(2) In the known edge rounding machine, if the workpiece A is pivoted in opposite directions from the vertical line V', the flat portion 3 of the machining face 1 is brought into contact with the grinding face 7 of the grinding sheet 5, so that scratches are produced on the surface of the flat portion 3. In order to obviate such inconvenience, it is arranged (as shown in FIG. 3a) such that the workpiece A is pivoted only in one direction with respect to the vertical line V' so as to be subjected to edge rounding. However, in this case, a problem is encountered in that the boundary 2b between the flat portion 3 and the edge-rounded portion 2a is obliquely deviated from the ideal line i as shown in FIG. 3b, thereby resulting in deterioration of the quality of the finished product of the workpiece A.
(3) Generally, profiles of the curved surfaces of the edge-rounded portions 2a of floppy disk heads are differently standardized according to the types of machines to be equipped with the floppy disk heads, respectively. Thus, in order to deal with various floppy disk heads formed with the edge-rounded portions 2a whose curved surfaces have different profiles, various kinds of elastic sheets 4 are required to be prepared therefor. In addition, each time the type and size of the workpieces A is changed, it is necessary to select a suitable elastic sheet 4 or determine optimum machining conditions of the workpieces A on a trial and error basis.
(4) When a plurality of the workpieces A are simultaneously subjected to edge rounding as shown in FIG. 2, respective grinding portions of the grinding sheet 5 have different circumferential speeds. Therefore, the finished products of the workpieces A vary in quality.